Yuko runs through the island in a complete panic when she witnesses the fight between Shuya Nanahara and Tatsumichi Oki. From her viewpoint, it seems that Shuya killed Tatsumichi, and she flees from the scene in terror. (After this she refer to Shuya as a “Demon Boy”.)
In the English-language manga she had a history of delusions, often faith-based, in which she believed killing Shuya was ‘God’s Will’, where she had seen a pastor about her delusions, and apparently had been at least somewhat mentally unbalanced as her file had spoke of her own doctor’s notes on her.

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